1. Field of the Invention
This invention relates to a device for increasing the efficiency of gaseous fuel fired furnaces. In one aspect, this invention relates to condensing gaseous fuel fired furnaces. In one aspect, this invention relates to the control of condensate formed during operation of the condensing furnace.
2. Description of Related Art
Residential furnaces are tested using a standard Annual Fuel Utilization Efficiency (AFUE) test to determine their efficiency with a standard comparison test that requires the furnace to be set up in a very specific configuration. The resulting efficiency number allows consumers to compare furnaces to each other on an equal basis; but it is not necessarily this efficiency at which the furnace will operate when installed in a home.
There are many factors that can affect the installed performance of gaseous fuel fired furnaces including geographic location, e.g., low altitude vs high altitude, ductwork installation and vent configuration. For example, as the length and number of elbows in the flue gas vent duct or conduit increase, the pressure drop increases, causing less air to be moved through the combustion process by the furnace combustion blower. When too much excess air is employed in the combustion process, the efficiency of the furnace decreases. The percent CO2 decreases and the flue gas temperature rises as less energy is transferred to the process air, i.e. the air to be heated, and more energy is exhausted out the flue gas vent. Typically, the blower is sized by the furnace manufacturer to allow for a maximum vent length (combination of elbows and vent length). However, in practice, most furnaces are not installed near the maximum vent length, resulting in wasted energy that could be utilized by the furnace.
Furnaces are currently installed at whatever vent length is required to reach the desired exit from the home or other building. This may be a very short run or it may approach the maximum distance allowed by the manufacturer. Generally, furnaces installed with short vent runs are not as efficient as furnaces installed with long vent runs.
There are essentially two types of gaseous fuel fired furnaces—non-condensing furnaces in which the flue gas generated by the combustion process is exhausted out the chimney or vertical metal vent and condensing furnaces in which the flue gas is typically exhausted through a flue gas vent conduit extending through a wall of the building. With non-condensing furnaces, which tend to be lower efficiency furnaces, a significant amount of heat is expelled with the exhausted flue gas. With condensing furnaces, which are typically power-vented and high efficiency furnaces (greater than about 88% efficiency), the amount of heat extracted from the process is so high that water vapor in the flue gas condenses. As a result, condensing furnaces typically require some means for handling the condensate so as to avoid corrosion that may be caused by the highly acidic condensate.
Flue dampers have been around for a long time and have been installed on furnaces and boilers (See U.S. Pat. No. 4,249,883). The primary purpose of a flue damper is to stop furnace off-cycle energy losses as conditioned air is lost out the flue gas vent during furnace off-cycle times. Thus, dampers are used to close the flue gas vent when the furnace is off and they are typically wide open when the furnace is on.
It is known that automatically restricting flue gas flow of non-condensing furnaces having a non-power vented environment by adjusting the flue damper based on feedback from the combustion process can be used to obtain proper combustion. In addition, U.S. Pat. No. 4,499,891 teaches the use of a baffle mechanism to restrict the flue of a natural draft furnace to slow down the flue products as a way of increasing furnace efficiency.